The invention relates generally to the field of position determination and, more particularly, to the field of position determination using GPS to precisely determine and maintain a position of a mobile platform such as a vehicle.
Position determination using conventional GPS (Global Positioning System) is well known in the art. GPS involves a system of satellites orbiting the earth and continuously broadcasting GPS signals which include both timing information for determining when the GPS signal was broadcast and data information as to the broadcasting satellites' orbital position. Position determination is accomplished using a receiver arranged at the position to be determined, for example the receiver can be in a movable platform such as a land, sea or air vehicle. The receiver makes ranging measurements between an antenna coupled to the receiver and each of at least four GPS satellites in view. These measurements are made from the timing information and the orbital position information from each satellite. Based on the four different GPS signals received, a fairly accurate position determination can be made.
In order to provide a more accurate position determination, other forms of GPS are well known, such as ordinary differential GPS, carrier phase differential GPS and dual frequency wide-laning GPS for example. While having different degrees of accuracy, all of these systems require that signals be received from at least four GPS satellites. In general, the requirement for four GPS satellite signals does not pose much of a problem for both air and sea vehicles, which operate in environments having an extremely limited number of obstructions. For land based vehicles, however, especially those operating in urban environments, the availability of four GPS signals is quite often limited. This can be due to obstructions caused by buildings and the like.
For a general discussion of the background and history of GPS position determination systems, reference can be made to U.S. Pat. No. 5,548,293 which discloses a known aircraft attitude determination method and system using four GPS antennas mounted on the aircraft in order to receive GPS signals of at least four GPS satellites, the specification of U.S. Pat. No. 5,548,293 being incorporated herein by reference.
Moreover, in particular when operating in an urban environment, the signals received by the vehicle are often subject to multipath errors. Multipath results from the numerous reflections the GPS signals undergo due to the structures in the area through which the vehicle is traveling. Multipath on a signal occurs when the receiver antenna picks up signals that have been reflected off of a surface between the emitter and receiver. GPS multipath can shift the computed position of the receiver significantly. For vehicles, the primary source of multipath is buildings, overhead foliage, and other vehicles. Multipath is often detected due to differences between GPS location and that computed using other sensors, however, the GPS receiver itself is often unaware of the multipath condition, thus the GPS positions are given undue weight in position determination, resulting in erroneous track of the vehicle. The multipath errors or noise substantially degrades the accuracy of the position determination.
There is therefore needed a position determining system and method which provides improved accuracy while minimizing the effects of multipath or other noise sources. These needs are met according to the present invention by providing a system and method using multiple GPS antennas and corresponding processors on a single vehicle in order to both maintain position information when fewer than four GPS satellites are visible and to reduce position errors by minimizing multipath induced errors using information combined from the multiple systems. While the present invention is described with respect to GPS signals, it should be readily understood that the system and method are applicable with any stable signal structure, such as from a satellite or other type of transmitter arranged at a known location.
In an embodiment of the present invention, multiple antenna/processor pairs are provided on a single platform, such as a vehicle. Position information is obtained by determining a cone angle between the various antenna/processor pairs and a single GPS satellite at a known orbital position. This cone angle is determined based on the differential path lengths between the single GPS satellite and at least two antennas on the vehicle. By providing a sufficient number of antenna baselines, along with a single GPS satellite, information on the attitude of the vehicle can be maintained over time given a known starting attitude. This is because a land vehicle is constrained in its movement along a specific axis, either forward or backward. Given this known constraint, the magnitude of the vehicle motion can therefore be determined from the Doppler shifts associated with a single satellite. The accuracy of this method is determined by the relative geometry of the antenna baselines and the satellite.
Advantageously, the position information obtained by determining the cone angle between the two antenna/processor pairs is implemented using a common clock between the processor receivers such that a single satellite is adequate. Of course, the inventive method is also applicable to receivers having independent clocks, in which case at least two satellites are required.
In another advantageous embodiment according to the present invention, the GPS signals or information received from the two antennas mounted on the vehicle can be combined to reduce noise effects, such as multipath. Correlation techniques can reduce noise effects by averaging out noise sources that do not correlate across the two antenna/receiver systems arranged on the vehicle. By minimizing the noise effects, the overall accuracy of the system can be improved.
By using multiple antennas on a single vehicle, it is also possible to assess the signal environment surrounding the vehicle. Based on the assessed signal environment, the present invention makes it possible to estimate the accuracy of the position determination. For example, multipath can introduce uncorrelated noise into the two receivers mounted on the vehicle. Alone, such noise cannot be detected by either receiver system. But, by comparing and correlating the two receivers, the system can determine the presence of a high multipath environment surrounding the vehicle. Accordingly, action can be taken to reduce the estimated position accuracy.
When the GPS position determination system is used in conjunction with other known position determining systems, it is possible to increase the position determination accuracy with the knowledge of the multipath condition by enabling the position determining algorithm to decrease its reliance (or weighting)on GPS information and rely more heavily on information from other position determining systems.
Any number of techniques can be used in accordance with the present invention to detect a multipath environment surrounding the vehicle. For example, multipath can be detected when correlating between the multiple antenna/receivers mounted on the vehicle by (a) comparing position, (b) correlating phase histories of a given GPS satellite, and/or (c) correlating drop-outs in received GPS satellite signals.
The present invention allows for a more accurate and more robust position determining system for vehicles, especially those moving in a restricted environment due to obstructions of the reference transmitters, such as the GPS satellites. Increasing the accuracy and availability of position information will, of course, provide numerous advantages in the field of vehicle tracking and its application for other purposes.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.